In a telephone system, ringing signals are usually required to actuate a ringer in a called telephone set, thereby giving audible indication of a telephone call. A typical ringing signal consists of a d.c. potential of between 45 and 55 volts and an a.c. potential of between 80 and 120 volts RMS being superimposed thereon. The a.c. potential may have a frequency of about 20 Hz which is typical, but frequency division multiplex ringing may require different frequencies in a range of 15 Hz and 100 Hz. In the past it was common for ringers in telephone sets to be connected between either of tip and ring subscriber loop conductors and ground to distinguish tip and ring parties. However recently, private line telephone service is increasingly popular and in the case of private line telephone service the ringer is usually connected across the tip and ring leads. In at least one operating telephone company, ringing signals are supplied from a floating ringing battery source, that is without any reference to ground.